Auto merge of #93028 - compiler-errors:const_drop_bounds, r=fee1-dead

Check `const Drop` impls considering `~const` Bounds

 This PR adds logic to trait selection to account for `~const` bounds in custom `impl const Drop` for types, elaborates the `const Drop` check in `rustc_const_eval` to check those bounds, and steals some drop linting fixes from #92922, thanks `@DrMeepster.`

r? `@fee1-dead` `@oli-obk` <sup>(edit: guess I can't request review from two people, lol)</sup>
since each of you wrote and reviewed #88558, respectively.

Since the logic here is more complicated than what existed, it's possible that this is a perf regression. But it works correctly with tests, and that makes me happy.

Fixes #92881

compiler/rustc_const_eval/src/transform/check_consts/qualifs.rs

@@ -4,11 +4,12 @@
 
 use rustc_errors::ErrorReported;
 use rustc_infer::infer::TyCtxtInferExt;
+use rustc_infer::traits::TraitEngine;
 use rustc_middle::mir::*;
 use rustc_middle::ty::{self, subst::SubstsRef, AdtDef, Ty};
 use rustc_span::DUMMY_SP;
 use rustc_trait_selection::traits::{
-    self, ImplSource, Obligation, ObligationCause, SelectionContext,
+    self, FulfillmentContext, ImplSource, Obligation, ObligationCause, SelectionContext,
 };
 
 use super::ConstCx;
@@ -145,44 +146,61 @@ impl Qualif for NeedsNonConstDrop {
         qualifs.needs_non_const_drop
     }
 
-    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, mut ty: Ty<'tcx>) -> bool {
-        // Avoid selecting for simple cases.
-        match ty::util::needs_drop_components(ty, &cx.tcx.data_layout).as_deref() {
-            Ok([]) => return false,
-            Err(ty::util::AlwaysRequiresDrop) => return true,
-            // If we've got a single component, select with that
-            // to increase the chance that we hit the selection cache.
-            Ok([t]) => ty = t,
-            Ok([..]) => {}
+    fn in_any_value_of_ty<'tcx>(cx: &ConstCx<'_, 'tcx>, ty: Ty<'tcx>) -> bool {
+        // Avoid selecting for simple cases, such as builtin types.
+        if ty::util::is_trivially_const_drop(ty) {
+            return false;
         }
 
         let Some(drop_trait) = cx.tcx.lang_items().drop_trait() else {
             // there is no way to define a type that needs non-const drop
             // without having the lang item present.
             return false;
         };
-        let trait_ref =
-            ty::TraitRef { def_id: drop_trait, substs: cx.tcx.mk_substs_trait(ty, &[]) };
+
         let obligation = Obligation::new(
             ObligationCause::dummy(),
             cx.param_env,
             ty::Binder::dummy(ty::TraitPredicate {
-                trait_ref,
+                trait_ref: ty::TraitRef {
+                    def_id: drop_trait,
+                    substs: cx.tcx.mk_substs_trait(ty, &[]),
+                },
                 constness: ty::BoundConstness::ConstIfConst,
                 polarity: ty::ImplPolarity::Positive,
             }),
         );
 
-        let implsrc = cx.tcx.infer_ctxt().enter(|infcx| {
+        cx.tcx.infer_ctxt().enter(|infcx| {
             let mut selcx = SelectionContext::new(&infcx);
-            selcx.select(&obligation)
-        });
-        !matches!(
-            implsrc,
-            Ok(Some(
+            let Some(impl_src) = selcx.select(&obligation).ok().flatten() else {
+                // If we couldn't select a const drop candidate, then it's bad
+                return true;
+            };
+
+            if !matches!(
+                impl_src,
                 ImplSource::ConstDrop(_) | ImplSource::Param(_, ty::BoundConstness::ConstIfConst)
-            ))
-        )
+            ) {
+                // If our const drop candidate is not ConstDrop or implied by the param env,
+                // then it's bad
+                return true;
+            }
+
+            if impl_src.borrow_nested_obligations().is_empty() {
+                return false;
+            }
+
+            // If we successfully found one, then select all of the predicates
+            // implied by our const drop impl.
+            let mut fcx = FulfillmentContext::new();
+            for nested in impl_src.nested_obligations() {
+                fcx.register_predicate_obligation(&infcx, nested);
+            }
+
+            // If we had any errors, then it's bad
+            !fcx.select_all_or_error(&infcx).is_empty()
+        })
     }
 
     fn in_adt_inherently<'tcx>(

compiler/rustc_middle/src/traits/mod.rs

@@ -566,7 +566,7 @@ pub enum ImplSource<'tcx, N> {
     TraitAlias(ImplSourceTraitAliasData<'tcx, N>),
 
     /// ImplSource for a `const Drop` implementation.
-    ConstDrop(ImplSourceConstDropData),
+    ConstDrop(ImplSourceConstDropData<N>),
 }
 
 impl<'tcx, N> ImplSource<'tcx, N> {
@@ -581,10 +581,10 @@ impl<'tcx, N> ImplSource<'tcx, N> {
             ImplSource::Object(d) => d.nested,
             ImplSource::FnPointer(d) => d.nested,
             ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
-            | ImplSource::Pointee(ImplSourcePointeeData)
-            | ImplSource::ConstDrop(ImplSourceConstDropData) => Vec::new(),
+            | ImplSource::Pointee(ImplSourcePointeeData) => Vec::new(),
             ImplSource::TraitAlias(d) => d.nested,
             ImplSource::TraitUpcasting(d) => d.nested,
+            ImplSource::ConstDrop(i) => i.nested,
         }
     }
 
@@ -599,10 +599,10 @@ impl<'tcx, N> ImplSource<'tcx, N> {
             ImplSource::Object(d) => &d.nested,
             ImplSource::FnPointer(d) => &d.nested,
             ImplSource::DiscriminantKind(ImplSourceDiscriminantKindData)
-            | ImplSource::Pointee(ImplSourcePointeeData)
-            | ImplSource::ConstDrop(ImplSourceConstDropData) => &[],
+            | ImplSource::Pointee(ImplSourcePointeeData) => &[],
             ImplSource::TraitAlias(d) => &d.nested,
             ImplSource::TraitUpcasting(d) => &d.nested,
+            ImplSource::ConstDrop(i) => &i.nested,
         }
     }
 
@@ -661,9 +661,9 @@ impl<'tcx, N> ImplSource<'tcx, N> {
                     nested: d.nested.into_iter().map(f).collect(),
                 })
             }
-            ImplSource::ConstDrop(ImplSourceConstDropData) => {
-                ImplSource::ConstDrop(ImplSourceConstDropData)
-            }
+            ImplSource::ConstDrop(i) => ImplSource::ConstDrop(ImplSourceConstDropData {
+                nested: i.nested.into_iter().map(f).collect(),
+            }),
         }
     }
 }
@@ -755,8 +755,10 @@ pub struct ImplSourceDiscriminantKindData;
 #[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
 pub struct ImplSourcePointeeData;
 
-#[derive(Clone, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
-pub struct ImplSourceConstDropData;
+#[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
+pub struct ImplSourceConstDropData<N> {
+    pub nested: Vec<N>,
+}
 
 #[derive(Clone, PartialEq, Eq, TyEncodable, TyDecodable, HashStable, TypeFoldable, Lift)]
 pub struct ImplSourceTraitAliasData<'tcx, N> {

compiler/rustc_middle/src/traits/select.rs

@@ -146,8 +146,8 @@ pub enum SelectionCandidate<'tcx> {
 
     BuiltinUnsizeCandidate,
 
-    /// Implementation of `const Drop`.
-    ConstDropCandidate,
+    /// Implementation of `const Drop`, optionally from a custom `impl const Drop`.
+    ConstDropCandidate(Option<DefId>),
 }
 
 /// The result of trait evaluation. The order is important

compiler/rustc_middle/src/traits/structural_impls.rs

@@ -120,12 +120,17 @@ impl<'tcx, N: fmt::Debug> fmt::Debug for traits::ImplSourceTraitAliasData<'tcx,
     }
 }
 
+impl<N: fmt::Debug> fmt::Debug for traits::ImplSourceConstDropData<N> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ImplSourceConstDropData(nested={:?})", self.nested)
+    }
+}
+
 ///////////////////////////////////////////////////////////////////////////
 // Lift implementations
 
 TrivialTypeFoldableAndLiftImpls! {
     super::IfExpressionCause,
     super::ImplSourceDiscriminantKindData,
     super::ImplSourcePointeeData,
-    super::ImplSourceConstDropData,
 }

compiler/rustc_middle/src/ty/util.rs

@@ -1041,6 +1041,42 @@ pub fn needs_drop_components<'tcx>(
     }
 }
 
+pub fn is_trivially_const_drop<'tcx>(ty: Ty<'tcx>) -> bool {
+    match *ty.kind() {
+        ty::Bool
+        | ty::Char
+        | ty::Int(_)
+        | ty::Uint(_)
+        | ty::Float(_)
+        | ty::Infer(ty::IntVar(_))
+        | ty::Infer(ty::FloatVar(_))
+        | ty::Str
+        | ty::RawPtr(_)
+        | ty::Ref(..)
+        | ty::FnDef(..)
+        | ty::FnPtr(_)
+        | ty::Never
+        | ty::Foreign(_) => true,
+
+        ty::Opaque(..)
+        | ty::Dynamic(..)
+        | ty::Error(_)
+        | ty::Bound(..)
+        | ty::Param(_)
+        | ty::Placeholder(_)
+        | ty::Projection(_)
+        | ty::Infer(_) => false,
+
+        // Not trivial because they have components, and instead of looking inside,
+        // we'll just perform trait selection.
+        ty::Closure(..) | ty::Generator(..) | ty::GeneratorWitness(_) | ty::Adt(..) => false,
+
+        ty::Array(ty, _) | ty::Slice(ty) => is_trivially_const_drop(ty),
+
+        ty::Tuple(tys) => tys.iter().all(|ty| is_trivially_const_drop(ty.expect_ty())),
+    }
+}
+
 // Does the equivalent of
 // ```
 // let v = self.iter().map(|p| p.fold_with(folder)).collect::<SmallVec<[_; 8]>>();